I’m holding the sunnies in front of a laptop LCD so you can see what’s going on.

2.Slowly rotate your gizmo while looking at the LCD (or rotate the glasses).

Can you see what’s happened here? The lenses look black! Keep rotating and light comes through again. You’ll find two angles that block the light from your LCD. Half way between these angles, the screen appears brightest.

3.Now try rotating your sunglasses while looking at the sky on a sunny day.

The sky won’t go completely black like the LCD but you should notice a difference.

4.Try the same thing while looking at the surface of the ocean, or a bitumen road, or a metal manhole cover.

5.(Diagram 1) Vertical waves on a rope can pass through a picket fence.

Imagine you are wiggling the rope up and down where the arrow is. These vertical waves pass straight through the slats.

6.(Diagram 2) Horizontal waves on a rope are blocked by a picket fence.

Now imagine you’re flicking the rope side to side. These horizontal waves get blocked by the slats.

what's going on?

Light is a type of wave - an electro-magnetic wave to be precise. But to understand what's happening, let's look at a different type of wave first.

If you wiggle a long piece of rope, you create waves that travel along the rope. If you wiggle the rope vertically (up and down), we'd call these vertically polarised waves. If you wiggle the rope horizontally (side to side), you'd create horizontally polarised waves.

In the first diagram above, the vertical waves pass straight through the slats in the picket fence. That's because the rope is free to move up and down in the space between the slats. If you wiggle the rope side to side however, the waves would are blocked by the slats.

So the fence allows vertically polarised waves to pass straight through while horizontally polarised waves are blocked. You could try this simple experiment with a piece of rope, or an extension cord. In our analogy, the rope represents the light waves and the fence represents the sunglasses.

Now let's say we only wiggle the rope vertically but we rotate the piece of fence so the slats are now horizontal. Can you see that vertical waves would now be blocked by the fence? And if we held the fence at an angle, can you see that only a small part of the wiggle would pass through the fence?

Now individual light waves are polarised too. And just like the picket fence in our analogy, polarised sunglasses only pass light that is polarised in one direction. But normal light is made up of many waves that are all polarised in different directions. That's why you won't notice anything if you rotate your sunglasses while looking at normal light (eg a light bulb or a television).

All the light waves from a liquid crystal display however, are polarised in exactly the same direction. As you rotate the sunglasses, more or less light passes through the lenses. It's the same as rotating the picket fence while wiggling the rope in our analogy. At the right angle, no light comes through at all. At 90 degrees to this angle, the maximum amount of light comes through. Rotate further and the light gets blocked again.

Now here's why polarised sunglasses are so handy - when light is reflected from a surface, it becomes partially polarised in a particular direction. That makes polarised lenses a good choice for any situation where there is a lot of glare. Fishermen particularly like polarised sunglasses because they reduce the glare from the water.

Look through your sunglasses on a sunny day and rotate your head and you'll notice all sorts of weird stuff happening. A road that has been patched up a lot, and metal manhole covers are really noticeable when you do this.

The sky is blue because the atmosphere scatters white sunlight. When light is scattered like this, it becomes partially polarised too. You can see that the blue sky is partially polarised by rotating your sunglasses while looking it.

Amazingly, bees and other insects can 'see' which way light from the sky is polarised and they use this ability to navigate … no sunnies required!